Todd H. Stievater

Thermo-optic tuning and switching in SOI waveguide Fabry-Perot microcavities

Compact silicon-on-insulator (SOI) waveguide thermo-optically tunable Fabry-Perot microcavities with silicon/air Bragg mirrors are demonstrated. Quality factors of Q=4,584 are measured with finesse F=82. Tuning is achieved by flowing current directly through the silicon cavity resulting in efficient thermo-optic tuning over 2 nm for less than 50 mW applied electrical power. The high-Q cavities enable fast switching (1.9 mus rise time) at low drive power (<10 mW). By overdriving the device, rise times of 640 ns are obtained. Various device improvements are discussed.

Integrated waveguide Fabry-Perot microcavities with silicon/air Bragg mirrors.

We demonstrate in-plane microfabricated Fabry-Perot cavities with cryogenically etched silicon/air distributed Bragg reflector (DBR) mirrors and integrated silicon-on-insulator rib waveguides. Several DBR configurations and cavity lengths were measured. Various devices exhibit Q=26963, FWHM=0.060 nm, finesse F=489, free spectral range FSR=81.7 nm, and DBR mirror reflectance R=99.4%. Thermo-optic tuning over 6.7 nm is also demonstrated.

A surface-normal coupled-quantum-well modulator at 1.55 /spl mu/m

We demonstrate a surface-normal coupled-quantum-well InGaAs-InAlAs electroabsorption modulator that provides optical modulation with a contrast ratio in excess of 1.5 at only 6 V. The device operates at 1.55 /spl mu/m and is based on a novel strain-balanced layer structure. The operating voltage is about two times lower than that of a conventional square quantum-well modulator that achieves a comparable contrast ratio.

Cryogenic etch process development for profile control of high aspect-ratio submicron silicon trenches

A cryogenic etch process using low temperature (T⩽−100°C) and SF6 and O2 gases is presented for fabricating high aspect ratio silicon microstructures, including photonic devices and micro- and nanoelectromechanical systems. The process requires only a single electron beam resist mask and results in open area etch rates of 4μm∕min. Various etch process parameters, including O2 flow, rf forward power, substrate temperature, and chamber pressure were studied, and the resulting effect on the etch quality was evaluated in terms of sidewall verticality and surface roughness. The optimized process uses low temperature (T=−110°C) and low chamber pressure (P=7mTorr) and enables sidewall verticality greater than 89.5° with roughness of 1–10nm. A silicon etch selectivity of 26:1 was obtained for 380nm thick electron beam resist. Using the optimized process, a silicon-on-insulator Fabry-Perot optical cavity with integrated rib waveguides and deeply etched silicon/air distributed Bragg reflector mirrors was fabricated...

In-plane microelectromechanical resonator with integrated Fabry–Pérot cavity

A silicon-on-insulator in-plane microelectromechanical resonator coupled to a high-Q (Q≈4,200), high finesse (FMax=265) optical Fabry–Perot microcavity is presented. The cavity utilizes high reflectance dry-etched silicon/air distributed Bragg reflectors. By suspending one of the Bragg mirrors to a microbridge resonator, the mirror can be displaced and the cavity is tuned. Using electrostatic actuation, bidirectional cavity tuning from −12.1to+17.0nm (29.1nm total range) is demonstrated near 1601nm wavelength. The device also enables measurement of thermal-mechanical noise with sensitivity better than 10fm∕Hz1∕2 and may find application in high resolution sensors.

Thermally induced nonlinearities in high-speed p-i-n photodetectors

Nonlinearities in the responsivity of high-speed p-i-n photodetectors at high photocurrents can limit the useful dynamic range in photonic systems. This letter describes a nonlinearity in the quantum efficiency in InGaAs photodetectors designed for applications at 1.5 /spl mu/m is due to significant ohmic heating of the intrinsic region. Measured changes in responsivity of about 10% at photocurrents of 12 mA are attributed to a thermal bandgap shift, based on comparisons with temperature dependent measurements and a model of ohmic heating.

Photonic microharp chemical sensors

We describe a new class of micro-opto-mechanical chemical sensors: A photonic microharp chemical sensor is an array of closely spaced microbridges, each differing slightly in length and coated with a different sorbent polymer. They are optically interrogated using microcavity interferometry and photothermal actuation, and are coupled directly to an optical fiber. Simultaneous measurements of the fundamental flexural resonant frequency of each microbridge allow the real-time detection and discrimination of a variety of vapor-phase analytes, including DMMP at concentrations as low as 17 ppb.

All-optical micromechanical chemical sensors

The authors describe experimental results from micromechanical resonators coated with a chemoselective polymer that detect chemical vapors from volatile organic compounds using all-optical interrogation. The shift in the resonant frequency of the gold microbeam is read out using photothermal actuation and microcavity interferometry. Response times of less than 5s are achieved for vapor concentrations as low as 60ppm using optical powers of a few megawatts.

Low-loss suspended quantum well waveguides.

We have used surface micromachining to fabricate suspended InGaAs/InGaAsP quantum well waveguides that are supported by lateral tethers. The average measured TE propagation loss in our samples is 4.1 dB/cm, and the average measured TE loss per tether pair is 0.21 dB. These measurements are performed at wavelengths in the optical L-band, just 125 nm below the quantum well band gap.

Measurement of thermal-mechanical noise in microelectromechanical systems

We report absolute measurements of thermal-mechanical noise in microelectromechanical systems. The devices are studied with an optical microcavity technique that has a resolution on the order of tens of femtometers per root hertz. The measured noise spectrum agrees with the calculated noise level to within 25%, a discrepancy most likely due to uncertainty in the effective dynamic mass of the vibrating bridge. These measurements demonstrate that thermal-mechanical noise can be the dominant noise source in actuated microelectromechanical devices.